Add direct messages (#52)

A DM is a Room with a new is_dm flag, not a separate model -- reuses
all the membership/message/WS plumbing Room already has instead of
duplicating it. The room's `name` (still required + globally unique)
is an internal, never-displayed token derived deterministically from
the two participants' sorted user IDs (dm_room_name), which makes
find-or-create a single indexed lookup and gets free race-condition
safety from the existing unique constraint -- a concurrent double-
start from both people just hits the same IntegrityError->retry-as-
lookup path create_room already established.

Both participants get the plain 'member' role (no owner/admin
distinction makes sense for a 1:1 DM), which incidentally reuses
every existing role gate to block add-member, room-settings edits,
and join-via-browse on a DM for free. update_room also gets an
explicit is_dm guard independent of that, since renaming a DM isn't
just a privacy concern -- it would silently corrupt the find-or-create
invariant. DMs are excluded from both Browse Rooms and the admin
portal's room listing (fully private, per scope).

GET /api/rooms/mine precomputes each DM's other participant (name,
avatar, presence) as dm_partner in one batched query, so the sidebar
can render a DM row without a fetch per row. Frontend: a new "Direct
Messages" sidebar section (searchable by partner name, not the
internal room name), clicking someone in the People list starts or
resumes a DM, and the chat header/composer/RoomInfoPanel all render
the partner's identity instead of a room name where it's a DM.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/Composer.tsx

@@ -22,6 +22,7 @@ import './Composer.css'
 interface ComposerProps {
   roomId: string
   roomName: string
+  isDm?: boolean
   members: RoomMember[]
   // #47: rooms this user belongs to, for the #roomname autocomplete --
   // deliberately the same list ChatPane already resolves message-display
@@ -89,7 +90,7 @@ function AttachMenu({ onPickPhoto, onPickFile, onClose }: AttachMenuProps) {
   )
 }
 
-export function Composer({ roomId, roomName, members, rooms, disabled, onSend }: ComposerProps) {
+export function Composer({ roomId, roomName, isDm, members, rooms, disabled, onSend }: ComposerProps) {
   const [value, setValue] = useState('')
   const [pendingImage, setPendingImage] = useState<{ id: string; previewUrl: string } | null>(null)
   const [pendingFile, setPendingFile] = useState<{ id: string; filename: string; size: number } | null>(
@@ -488,7 +489,13 @@ export function Composer({ roomId, roomName, members, rooms, disabled, onSend }:
             }}
             onSelect={handleSelectionChange}
             onKeyDown={handleKeyDown}
-            placeholder={disabled ? (online ? 'Connecting…' : "You're offline") : `Message #${roomName}`}
+            placeholder={
+              disabled
+                ? online
+                  ? 'Connecting…'
+                  : "You're offline"
+                : `Message ${isDm ? roomName : `#${roomName}`}`
+            }
             spellCheck
           />
           {mentionQuery && mentionMatches.length > 0 && (